Apparatus for recording digital signals superposed with controlled frequency components

ABSTRACT

When modulating by an interleaved NRZI technique by inserting one bit per every m bits of input data series, the frequency characteristics of bit rows varying by the polarity (&#34;0&#34; or &#34;1&#34;) of the bit to be inserted are compared and the bit row closer to the desired frequency characteristic is selected as the output series, so that recording is effected by controlling the frequency characteristics of the digital signal.

This application is a continuation of now abandoned application, Ser. No. 08/124,637, filed on Sep. 22, 1993.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a recording apparatus for recording digital signals, such as a digital video cassette recorder (VCR).

2. Description of the Prior Art

In a magnetic recording apparatus, such as a VCR, when reproducing, as the head goes off the track, the head output is lowered, and the error increases, so that an accurate picture cannot be reproduced. It is therefore important that the head traces the intended track accurately, that is, to keep the head tracking. In the digital VCR for home use, in particular, the tracks are narrow in order to record long programs, and it is necessary to keep the head tracking more accurately.

As the means for detecting the deviation of the head from the track, using pilot signals between tracks, the crosstalk of the pilot signal from the preceding track is compared with that of the succeeding track and a determination is made as to whether the head tracking has deviated to the preceding side or to the succeeding side.

The recording signals for this purpose are modulated when recording so as to have the frequency characteristics of three patterns F0, F1, F2 as shown in FIG. 22. In the F0 pattern, frequencies f₁ and f₂ possess portions of small frequency components, that is, notch portions. In the F1 pattern, frequency f1 possesses a portion having a larger frequency component, that is, a pilot signal, while frequency f₂ possesses a notch portion. In the F2 pattern, frequency f₁ possesses a notch portion, and frequency f₂ has a pilot signal.

The recording signals are modulated so that the patterns may be in the order of F0, F1, F0, F2 as shown in FIG. 23, and recorded. When reproducing the F0 pattern, by the crosstalk of pilot signals from the F1 and F2 patterns of adjacent tracks, there is a peak in frequency components of f₁ and f₂. When the head is deviated from the center of the F0 pattern and is shifted to the F1 pattern side, the crosstalk of the pilot signal from, the F1 pattern becomes larger than the crosstalk talk from the F2 pattern, and therefore the frequency components of f₁ of reproduced signal increase, while the frequency components of f₂ decrease. Thus, by comparing the frequency components of f₁ and f₂ of the reproduced signals of the F0 pattern, the deviation of head tracking can be detected, and a correct tracking is realized.

Hitherto, the patterns of F0, F1, F2 are formed by controlling the linkage of "0" and "1" in the binary series to be recorded. This method is explained below. First, the input data is divided by every m bits (m: an even number), and a "0" bit is added to the beginning of the m bits to enter a pre-coder to be modulated by an interleaved NRZI (Non Return to Zero Invert) technique. Similarly, a "1" bit is added to the beginning of the m bits of the input data to enter the precoder to be modulated by the interleaved NRZI technique. The characteristic of the pre-coder is expressed by formula (1) below, and it is utilized for known partial response detection when decoding, and moreover when the polarity of the bit to be inserted is inverted, the inversion of polarity is propagated as shown in formula (1), and it causes an increase to the change of frequency characteristics by the change of polarity of the bit to be inserted. The bit of "0" or "1" to be inserted is hereinafter called a special bit (SB).

    f.sub.k =g.sub.k +f.sub.k-2 (+is exclusive OR)             (1)

where {g_(k) } is a pre-coder input data series, and {f_(k) } is a pre-coder output data series. Extracting the frequency components of the pre-coder output, the frequency components are compared between the pre-coder output when the SB is "0", and the pre-coder output when the SB is "1", and the precoder output closer to the desired frequency characteristic is used as the output of the recording apparatus, so that the output data series having the desired frequency characteristic is obtained.

FIG. 2 shows examples of an input data series by the interleaved NRZI technique at m=10, SB A, and output data series. As the polarity of SB A is inverted, the polarity of the second bit ahead of SB A is inverted according to formula (1). As the polarity of the second bit ahead of SB A is inverted, the polarity of the second bit ahead of the second bit ahead of SB A is inverted. In this way, as the polarity of SB A is inverted, the polarity of every odd-number bit counting from SB A is inverted. This Inversion continues until the odd-number bit from SB A is a new SB, that is, SB C. In other words, by inverting the polarity of a certain SB, the polarity of the odd-number bit counting from that SB is inverted m times. The bit of (m+1) counting is a new SB C, and this inversion is not propagated. The bit inverted as SB A is inverted are m+1 bits indicated by bullet mark.

By the interleaved NRZI and the propagation of change of SB polarity, changes of frequency characteristics by an SB polarity change are increased.

Conventionally, the frequency components were compared between the bit row of m+1 bits by interleaved NRZI modulation with "0" added as an SB to m bits, and the bit row of m+1 bits by interleaved NRZI modulation with "1" added as SB to m bits, and by recording m+1 closer to the frequency component of the desired frequency, the desired frequency component of the recorded signal was controlled. As the means for extracting frequency components, a Fourier transform is used.

SUMMARY OF THE INVENTION

It is hence a primary object of the invention to realize a circuit for controlling the frequency characteristics of the digital the signals by varying the polarity of SB to be inserted due to the change of frequency characteristics of the bit varying in polarity by changing the bit to be inserted when modulating by the interleaved NRZI technique by inserting one bit in every m bits, by a simple circuit (small-scale circuit).

To achieve the above object, the invention presents a recording apparatus comprising: a bit extracting means for extracting inverted m+1 bits of a first series of bits obtained by the interleaved NRZI modulation technique of an input data series added to one "0" bit per every m bits (m is an even number equal to or greater than 2) and a second series of bits obtained by the interleaved NRZI modulation technique of the input data series added to one "1" bit per every m bits; a frequency component extracting means for extracting at least two specified frequency components of the bit extracting means; a output selection means for producing an output bit series depending on the size of the frequency component, and a recording means for recording the output bit series on a magnetic tape.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of an embodiment of the present invention.

FIG. 2 is an explanatory diagram of the interleaved NRZI modulation technique.

FIG. 3 is a block diagram of a bit extracting circuit 2.

FIG. 4 is a block diagram of a pre-coder.

FIG. 5 is a block diagram of another pre-coding portion.

FIG. 6 is a block diagram of an inverting circuit.

FIG. 7 is a block diagram of an output selecting circuit 5.

FIG. 8 is a block diagram of a pre-coder 14.

FIG. 9 is a block diagram of another embodiment of the present invention.

FIG. 10 is a block diagram of a bit extracting circuit 19.

FIG. 11 is a block diagram of an output selecting circuit 20.

FIG. 12 is a block diagram of another bit extracting circuit.

FIG. 13 is a block diagram of a bit dividing circuit 24.

FIG. 14 is a block diagram of a judging circuit 4.

FIG. 15 is a block diagram of an IIR type digital filter 26 or 27.

FIG. 16 is a block diagram of digital filter 26.

FIG. 17 is a block diagram of other constitution of the digital filter 26.

FIG. 18 is a block diagram of a representative value generating circuit 38a.

FIG. 19 is a block diagram of a different constitution of the digital filters filter 26.

FIG. 20 is a frequency characteristic diagram of the digital filter.

FIG. 21 is a frequency characteristic diagram of the digital filter.

FIG. 22 is a frequency characteristic diagram of the recording signal.

FIG. 23 is a track pattern diagram.

DESCRIPTION OF THE EMBODIMENTS

FIG. 2 shows the relationship between the input data series and pre-coder output series in the interleaved NRZI modulation technique. In every m bits of the input data series, an SB is inserted by one bit. By the pre-coder,-the input data series is pre-coded according to formula (1). Therefore, when the polarity of the SB is inverted, the odd-number bit counting from the SB is inverted. When the polarity of SB A is inverted, the odd-number bit from SB A is inverted, but the bits after SB C are determined by SB C, and the polarity of the bit after SB A by SB A influences only m+1 bits indicated by the bullet marked bit in the pre-coder output series In FIG. 2. The circle mark bits do not depend on the polarity of SB A.

Hitherto, in the unit of m+1 bits starting from SB, the frequency component was extracted and the SB was determined. According to this method, when the polarity of the SB A is inverted, among m+1 bits to be noticed, the polarity is inverted in m/2+1 bits indicated by the bullet marks, but the polarity is not inverted in m/2 bits indicated by the circle marks. That is, m/2 bits are fixed bits which are not inverted by the polarity of SB A. By the presence of these fixed bits, the change of the frequency component by inversion of the SB is blocked, which results in interference for obtaining a bit row possessing a desired frequency characteristic. In m/2 bits in the next word, there are bits inverted in polarity by inversion of the polarity of SB A, but this interval is not taken into consideration for the calculation of frequency components. Therefore, by performing the calculation of the frequency components only in the SB and the bits Influenced by this SB, the change of the frequency characteristic by the polarity change of the SB can be increased, so that the notch or pilot signal of excellent characteristic may be generated.

This is to extract the bits influenced by the SB which are extracted in the pre-coded series, but it is also possible to extract the bits to be influenced by the SB preliminarily from the series to be pre-coded or the input data series, and pre-code the selected bit row.

By calculating the frequency components of the bullet marked bits only in FIG. 2, the number of bits to calculate the frequency components is m+1 bits, which is same as in the prior art, and the circuit scale or frequency component calculation is unchanged. Besides, by inversion of the SB, all of bits used in the frequency component extraction are inverted, and therefore the pre-coded data when SB is "0" can be determined by inverting the pre-coded data when the SB is "1", and it is not necessary to determine the pre-coded data both when the SB is "0" and when the SB is "1", so that the circuit scale may be further curtailed.

One of the embodiments of the Invention is described below with reference to accompanying drawings. FIG. 1 is a block diagram of an embodiment of the present invention. In FIG. 1, numeral 1 is an A/D converter; element 2 is a bit extracting circuit; elements 3a and 3b are pre-coders; element 4 is a judging circuit; element 5 is an output selecting circuit; element 6 is a recording amplifier; element 7 is a recording head, and element 8 is a magnetic tape. An input video signal is sampled and quantized In the A/D converter 1, and is converted into a digital bit series {e_(k) }. This bit row is divided In every m bits, and an SB is inserted between them, and the interleaved NRZI modulation technique is effected, and the bit series to be recorded {d_(k) } is obtained, and by noticing a certain SB, in the bit extracting circuit 2, in order to determine this SB, m/2 even-number bits among next m bits from the place of insertion of the SB of notice in the output bit series of the A/D converter 1, and m/2 odd-number bits out of the next m bits are output to the pre-coders 3a and 3b. The bit row of m bits produced from the bit extracting circuit 1 comprises the bits influenced by the polarity of the SB when modulated by the interleaved NRZI technique. These bit rows (a_(n) (n=0, 1, . . . , m-1)) are prefixed by the SB by the pre-coders 3a and 3b, modulated by the Interleaved NRZI technique, and output to the Judging circuit 4 as bit rows of m+1 bits. Herein, the pre-coder 3a adds "0" as the SB, and produces bit rows b_(n) (n=0, 1, . . . , m), while the pre-coder 3b adds "1"0 as the SB, and produces bits b_(n), (n=0, 1, . . . , m). In the judging circuit 4, comparing the frequency characteristics of b_(n) and b_(n'), the bit row closer to the desired frequency characteristic is judged, and the determined to output SB is the output selecting circuit 5. In the output selection circuit 5, the SB output by the Judging circuit 4 is inserted in every m bits of output series {e_(k) } of the A/D converter 1, and the interleaved NRZI modulation technique is performed, and the record bit row {d_(k) } is sent to the recording amplifier 6. In the recording amplifier 6, the record bit row {d_(k) } is converted into the recording current, and is output to the recording head 7. Using the recording head 7, the record bit row is recorded on the magnetic tape 8.

A block diagram of the bit extracting circuit 2 is shown in FIG. 3. The bits changed in polarity by the SB polarity are bullet marked bits in the input data series shown in FIG. 2. Accordingly, the input data series {e_(k) } is fed into a shift register 9, and when the next bit from the place for inserting the SB at the beginning comes, the even number bits of next m bits from the SB inserting place, and the odd number bits of second bit therefrom are produced. These bits are a_(n).

In the pre-coders 3a and 3b, the input bit rows are pre-coded. The pre-coders 3a and 3b produce the input SB as b₀ as shown in FIG. 4, and operate a0 by exclusive OR with SB in an EXOR 10, and produce b1. This output, that is, b1 is operated by exclusive OR with al in the EXOR 10, and b2 is output. Thus, a_(p) is produced as b_(p+1) by exclusive OR with b_(p) (p=0, 1, . . . , m-1). At this time, the pre-coder 3a receives the SB as 0, and produces bit row b_(n). The pre-coder 3b receives the SB as 1, and produces bit row b_(n').

Herein, between the output bit row of the pre-coder 3a when the SB is 0, and the output bit row of the pre-coder 3b when the SB is 1, as mentioned above, there is a relationship in which all bits are mutually Inverted, and hence, without using two pre-coders as in FIG. 1, by inverting the output b_(n) of the pre-coder 3a by an inverting circuit 11 as shown in FIG. 5, the same as the output b_(n) of the pre-coder 3b is obtained. Likewise, by generating the bit row b_(n) ' by using the pre-coder 3b, the bit row b_(n) may be generated by inverting it. Or, by using one pre-coder in time sharing, b_(n) and b_(n') can be generated. Moreover, since b₀ is delivered to the judging circuit but b₀ is always 0 and b_(n') is always 1, they may be generated in the Judging circuit 4 without producing outputs.

A block diagram of an inverting circuit 11 is shown in FIG. 6. An inverting element 12 produces 1 when the input bit is 0, and produces 0 when the input bit is 1. By feeding the bits of the bit row b_(n) into the inverting element 12, all bits are output in inverted form. The output bit row is a same bit row as b_(n'). Thus, b_(n) and b_(n') are generated.

The b_(n) and b_(n') are fed into the Judging circuit 4, and the bit row closer to the specified frequency characteristic is judged, and the determined output SB is. The determined SB is fed into the output selecting circuit 5. In the output selecting circuit 5, the bit series {e_(k) } is entered. A block diagram of the output selecting circuit 5 is shown in FIG. 7. The bit series {e_(k) } is fed into a delay circuit 13, and it is delayed until the SB is Judged in the Judging circuit 4. The output of the delay circuit 13 is fed into a pre-coder 14, and the determined SB is inserted in every m bits, and the pre-coded output is produced. The pre-coder 14 is composed, for example, as shown in FIG. 8, and the determined SB of the judging circuit 5 is inserted by a switch 15 in every m bits of the input bit series {e_(k) }. The bits delayed by the delay circuit 16 from the output series of the pre-coder 14, and the output bits of the switch 15 are operated by exclusive OR in an EXOR 17, and the result is produced as the output of the pre-coder 14. Herein, the delay circuit 16 and EXOR 17 are the circuits for realizing formula (1). The delay circuit 16 delays by one clock. The record bit series {d_(k) } which is the output of the output selecting circuit 5 is converted into a recording current by the recording amplifier 6, and recorded on the magnetic tape 8 by the recording head 7.

Incidentally, the record bit series {d_(k) } may be also produced as the record bit series {d_(k) } by restructuring the pre-coded outputs b_(n) and b_(n') of the pre-coders 3a and 3b according to the determined SB, without pre-coding the {e_(k) } newly.

In the foregoing description, the SB is judged by extracting the bits, that is, a_(n) of the input data series changed in the polarity by varying the polarity of SB, in the bit extracting circuit 2. By contrast, the SB may be preliminarily inserted as 0, and pre-coded, and bits changing in the polarity in the pre-coded series can be extracted. A block diagram at this time is shown in FIG. 9. The input data series is fed into a pre-coder 18, the SB is inserted as 0, and the Interleaved NRZI modulation technique is performed. The output of the pre-coder 18 is fed to a bit extracting circuit 19, and bits changing in polarity by the change of SB polarity are extracted. The extracted bits are the same bit rows as the output, or b_(n) of the pre-coder 3a when the SB is 0. The output of the bit extracting circuit 19 is fed into the inverting circuit 11, and the pre-coded output b_(n') when the SB is 1 is produced. Both b_(n) and b_(n') are fed into the Judging circuit 4, and the bit row closer to the desired frequency characteristic is judged, and the determined SB is produced. The determined SB is fed into an output selecting circuit 20. In the output selecting circuit 20, from the determined SB and output series of the pre-coder 18, the recording bit series is delivered. The recording bit series is converted into a recording current by the recording amplifier 6, and recorded on the magnetic tape 8 by the recording head 7.

The pre-coder 18 is the same circuit as the pre-coder 14. However, the "0" bit is inserted as the SB. An example of the bit extracting circuit 19 is shown in FIG. 10. The bits to be inverted as the SB is inverted in the pre-coder output series b_(n) are, as shown in FIG. 2, m+1 bits including the SB of the odd number counting from the SB. The input pre-coder output series is fed into a shift register 21 of 2m+1 stages, and when the SB comes to the beginning of the shift register 21, m+1 bits are delivered in every 2 bits. This is the bit row pre-coded when the SB is 0, that is b_(n') and it is fed in the judging circuit 4, and is also fed into the inverting circuit 11, thereby generating b_(n'). The output b_(n') of the inverting circuit 11 is also input to the judging circuit 4. An example of output selecting circuit 20 is shown in FIG. 11. The output series of the pre-coder 18 in FIG. 9 is fed into the delay circuit 22, and is delayed until the SB is determined in the Judging circuit 4. The SB determined to be the output series of the delay circuit 22 is fed to an EXOR 23. That is, what is fed into the output selecting circuit 18 is the pre-coder output series when the SB is "0", and nothing is delivered when the determined SB corresponding to the output bit is "0", and when the determined SB is "1", it is inverted and output.

Instead of the record bit series {d_(k) }, the determined SB may be inserted into the input data series {e_(k) }, and pre-coded, and the record bit series {d_(k) } may be obtained.

The bit extracting circuit 2 may be composed without using the shift register. A block diagram of such a bit extracting circuit 2 is shown in FIG. 12.

The data series {e_(k) } entered in m-bit parallel data is fed into a bit dividing circuit 24, and is divided into odd-number bits and even-number bits from the beginning. An example of the bit dividing circuit 24 is shown in FIG. 13. The odd-number bits divided by the bit dividing circuit 24 are fed into the delay circuit 21. The output bit row of a delay circuit 25 and the even-number bit output of the bit dividing circuit 24 are nothing but a_(n). In such a constitution, the circuit scale may be further reduced for the data series of parallel input.

In such a constitution, by extracting the bit row changing in the polarity by varying the SB polarity, and determining the SB by the comparison of the frequency components of the bit rows, bit series of larger notches or larger pilot signals may be generated and recorded. In this method, the number of bits of the bit row used In the Judgement of SB is unchanged, that is, m+1 bits, and the recording apparatus may be composed in a similar circuit scale.

To Judge the SB, hitherto, the frequency component was extracted by using a Fourier transform. For a Fourier transform, however, it was necessary to multiply the input series by the sine and cosine waveforms as the reference. As known well, the multiplier is large in circuit scale, which caused an increase in the cost of the circuit for extracting the frequency component. In addition, to determine the frequency component, the mean square of the determined sine component and cosine component must be calculated. It is also large in the circuit scale, leading to an increase of the cost of the circuit for extracting the frequency component.

To solve these problems, it has been found to be possible to extract the frequency components by using a digital filter. FIG. 14 shows a block diagram of the judging circuit 4 using digital filters for extracting the frequency components for judging the SB. In digital filters 26 and 27, the frequency components of the input bit series are extracted by using the digital filters of IIR the (Infinite impulse Response) type. Herein, notches are provided at two frequencies f₁ and f₂, and the f₁ frequency components of b_(n) and b_(n') are determined in the digital filter 26, and the f₂ frequency components of b_(n) and b_(n') are determined in the digital filter 27. The f₁ frequency component and f₂ frequency component of b_(n) determined in the digital filters 26 and 27 are added in an adder 28a, and fed into a comparator circuit 29. Similarly, the f₁ frequency component and f₂ frequency component of b_(n') are added in an adder 28b, and fed into the comparator circuit 29. In the comparator circuit, the frequency components of b_(n) and b_(n') , are compared, and the smaller SB is delivered as the determined SB. The determined SB is fed into the digital filters 26 and 27 in order to match with the internal data of the digital filters. When adding the pilot component to the frequency of f₀, before input of b_(n) and b_(') to the digital filters 26 and 27, sin 2πf₀ t is added by the adder 25a and adder 25b, the series of the frequency characteristic possessing a pilot component and also having a notch can be selected. In the adder 25a, b_(n) and sin 2π are added, and in the adder 25b, b_(') and sin 2π are added. Herein is shown a block of judging circuit for selecting the series in which notches are provided in the frequencies of f₁ and f₂ and there is a pilot signal in the frequency of f₀, but the selection of a bit series having notches in more frequencies is possible with the parallel connection of a digital filter for extracting the frequency component with the digital filters 26 and 27. The pilot frequency is f₀, but it may be the same frequency as f₁ or f₂.

A block diagram of general IIR type digital filters is shown in FIG. 15. The outputs of digital filters are delayed in a delay circuit 32, a coefficient is multiplied in a multiplier 31, and input data series is added, and the sum is output. By varying the number of stages of the delay circuit 32 and the coefficient of the multiplier 31, it is known that a filter possessing an arbitrary frequency characteristic can be obtained. The digital filter 26 and digital filter 27 extract the frequency components of f₁ and f₂ according to the principle of the IIR type digital filter. In this constitution, a delay circuit 32 is an element for delaying the bit clock of 1 bit.

A block diagram of the digital filter 26 is shown in FIG. 16. An adder 33a, delay circuits 32, 34 and 34a and multiplier 31a compose an IIR type digital filter having the peak at the frequency of f₁ to b_(n). The output of the adder 33a is combined with the sum of the frequency components hitherto recorded in a register 37a by the adder 36a, and an absolute value is obtained in the absolute value circuit 35a, and the f₁ frequency component of b_(n) is delivered. Similarly, an adder 33b, delay circuits 32, 34, 34b, and multiplier 31b compose an IIR type digital filter having the peak at the frequency of f₁ to b_(n'), and produce an output. The output of the adder 33b is combined with the sum of the frequency components hitherto recorded in a register 37b by the adder 36b, and an absolute value is obtained in the absolute value circuit 35b, and the frequency component of f₁ of b_(') is produced.

In this way, the frequency components of b_(n) and b_(') are calculated and produced. The delay component of the digital filter when the SB is 0 is delayed in the delay circuit 34a, and when the SB is 1 in the delay circuit 34b. Accordingly, when the SB is determined by the frequency components of outputs b_(n) and b_(n'), if the determined SB is 0, the content of the delay element 34a is copied in the delay circuit 34b. If the determined SB is 1, to the contrary, the content of the delay element 34b is copied in the delay circuit 34a. Similarly, the contents of the registers 37a and 37b are copied according to the determined SB. Thus, regardless of the determined SB, the matching of digital filters is achieved.

In FIG. 16, the digital filter 26 is explained, whereas the digital filter 27 is composed by constructing the IIR type digital filter having the peak at f₂ by varying the number of stages of the delay circuit 32 in FIG. 15 and coefficients of the multipliers 31a and 31b.

In this way, the frequency components can be operated upon by using the digital filters. This method is not practical because lots of calculations are necessary in each bit. Hence, without having to calculate the frequency components in every bit, by determining a representative value in every specific bits, and determining the frequency component of the representative value, the frequency components of bit row can be determined. A block diagram of the digital filter 26 in this case is shown in FIG. 17. The input b_(n) is input to a representative value generating circuit 38a, and the input b_(') is input to a representative value generating circuit 38b, and the representative value of every k bits (k is a natural number) is generated and produced. The adder 33a, delay circuits 40, 39, and 39a, and multiplier 31a compose an IIR digital filter having a peak at f₁, and the frequency component of f₁ of the representative value of the input b_(n) is produced. Similarly, the adder 33b, delay circuit 40, 39 and 39b, and multiplier 31b composed an IIR type digital filter having a peak at f₁, and the frequency component of f₁ of the representative value of the input b_(n') is produced. The output of the adder 33a is combined with the value of the register 37a, the absolute value is obtained by the absolute value circuit 35a, and produced as the frequency component of f₁ of b_(n). Similarly, the output of the adder 33b is combined with the value of the register 37b, and the absolute value is obtained by the absolute value circuit 35b, and is produced as the frequency component of f₁ of b_(n'). As in the digital filter 26 shown in FIG. 16, when the SB is determined, the value determined by the delay circuits 39a and 39b, or registers 37a and 37b is copied in the other delay circuit or register.

As an example of representative value produced by the representative value generating circuit, DSV (digital sum variation) may be employed. A block diagram of the representative value generating circuit for generating DSV as a representative value is shown in FIG. 18. The DSV is obtained by integrating "0" of bit row as -1, and "1" as +1. The DSV is stored In the register 42. In the adder 41, converted into the input bit +1 or -1, it is combined with the DSV stored In the register 41, and the sum is stored a register 42. After addition of k-th bit, the DSV of the calculation result is combined with the stored DSV, and produced as output. The DSV generated here is the DSV corresponding to the input series. After the SB is determined, the value of the register 42 of the determined representative value calculating circuit is copied in the register 42 of the representative value calculating circuit.

In this circuit composition, as compared with the circuit composition in FIG. 16, the delay circuit and multiplier are required only by 1/k for extracting frequency components of the same frequency, and the circuit scale is reduced, and the operating speed can be slowed down, so that the energy may be saved.

In the circuit composition in FIG. 17, if the coefficients of the multipliers 31a and 31b of the final stage are 1, and the coefficients of the other multipliers 31a and 31b are 0, then a block diagram of digital filter 26 is shown in FIG. 19.

Of the input bit row b_(n') the representative value is generated and produced by the representative value generating circuit 38a. The representative value is fed into a digital filter composed of adder 33a, and delay circuits 43 and 44a. The output of the adder 33a is fed into the absolute value circuit 25a, and is produced as the f₁ frequency component of b_(n). Similarly, of the input bit row b_(n'), the representative value is generated and produced in the representative value generating circuit 38b. The representative value is fed into a digital filter composed of adder 33b and delay circuits 43 and 44b. The output of the adder 33b is fed into the absolute value circuit 25b, and produced as frequency components of f₁ of b_(n').

The frequency characteristic of the digital filter in FIG. 19 is shown in FIG. 20. The delay time of the delay circuit 43 is set so that the frequency to be extracted, or f₁, may be the lowest frequency of peak. When generating notches at plural frequencies, by varying the delay time of the delay circuit 43, it is possible to vary the frequency for generating notches. Such digital filters are arranged in parallel as many as the number of notches, and the sums of the frequency components are compared, and the SB is determined.

Reviewing the frequency characteristics in FIG. 20, not only f₁ which is the desired frequency, but also the DC component at which the frequency is 0, and component of frequency of a common multiple of f₁ are produced by the digital filter in FIG. 19. Accordingly, when generating notches in two frequencies f₁ and f₂, by using the digital filter in FIG. 19 at the frequency of the greatest common measure of f₁ and f₂, the notches at frequencies f₁ and f₂ can be composed of one digital filter. Supposing the ratio of frequencies f₁ and f₂ to be 2:3, the frequency characteristic may be as shown in FIG. 21 by setting the fundamental frequency of digital filter to be f₁ /2, and it is possible to, set notches at two frequencies by using only one digital filter, which contributes to a reduction of circuit scale. 

What is claimed is:
 1. A recording apparatus comprising:a bit extractor, said bit extractor extracting even-numbered bits from a series of m bits of an input data series and odd-numbered bits from a next series of m bits of the input data series, where m is an even number of 2 or larger, and combining the even-numbered bits extracted from said series of m bits of the input data series with the odd-numbered bits extracted from said next series of m bits of the input data to obtain a new series of m bits; a first pre-coder, said first pre-coder adding one "0" bit to said new series of m bits and modulating the added result by NRZI modulation to obtain a first series of m+1 bits; a second pre-coder, said pre-coder adding one "1" bit to said new series of m bits and modulating the added result by NRZI modulation to obtain a second series of m+1 bits; an output selector, said output selector comparing frequency characteristics of respective said first and second series of m+1 bits with a predetermined frequency characteristic and selecting from said first and second series of m+1 bits one whose frequency characteristic is closer to said predetermined frequency characteristic to obtain output bit series; and recording means for recording said output bit series on a recording medium.
 2. A recording apparatus according to claim 1, wherein the input data series is a series of parallel m bits, and wherein said bit extractor comprises a delay for delaying odd-number bits of each parallel m bits of the input data series, and a combiner for combining output bits from the delay and even-numbered bits of next parallel m bits of the input data series. 